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INCOME TAXES: (Details 2)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Y
jurisdiction
Dec. 31, 2010
Dec. 31, 2009
Reconciliation of the statutory income tax rate to the effective tax rate            
Expected tax (as a percent)       30.00% 30.00% 30.00%
Effect of income taxed at a rate other than the statutory rate (as a percent)       4.30% 5.30% 8.30%
Percentage depletion       (4.00%) (4.30%) (5.00%)
Other permanent differences (as a percent)       1.20% 1.80% 2.10%
Peru tax on net income deemed distributed (as a percent)       1.30% 2.00% 2.00%
Mexican tax on dividends (as a percent)         0.40% 1.60%
Increase (decrease) in unrecognized tax benefits for uncertain tax positions (as a percent)         4.60% (3.90%)
Outside basis difference (as a percent)       2.10%    
Prior period adjustment (as a percent)       (3.50%) (4.50%) (1.40%)
Other (as a percent)       0.60% 0.40% (0.20%)
Effective income tax rate (as a percent)       32.00% 35.70% 33.50%
Number of jurisdictions where company files income tax returns       3    
U.S. statutory rate (as a percent)       35.00% 35.00% 35.00%
Mexican tax rate (as a percent) 28.00% 29.00% 30.00% 30.00% 30.00% 28.00%
Duration for which statutory tax rates for Peru and Mexico are considered to determine effective tax rate (in years)       3